Sunday, February 27, 2011

Weekly summaries include annual assessments

27 Feb '11: At the end of every week, I enter the current readings for everything, and I also put in the reading of exactly a year ago. This means I get an annual update every week.
This diagram is compiled in March, showing
completed graph including February
and deducting power use of DHW and underfloor
circulating pump
  My initial yardstick was 8,500 kWh total consumption for the house for both the years of Aug'07-Aug'08 and Aug'08-Aug'09. I know not the breakdown of the figures, and we didn't have PV then, but let's assume that the house consumption was 3,000 and the heat pump 5,500.

  I moved my yardstick assumption to Oct-Oct as this is the start of the winter heating season and it is the date of our Photovoltaic installation.  For 2009-2010 with the help of the PV roof and more careful management of the GSHP, we managed to get down to a lowest ever of 6,080 kWh for the house and 3,996 kWh for the GSHP.
    As the winter progressed, we remember how cold November and December 2010 turned out to be, and these figure became more and more remote, reaching a new 'bad' of 6,302 kWh (house) and 4,245 kWh (GSHP)
   With the warmer weather of Spring 2011, the annual figures are getting better and better and by next month, I am predicting the annual figures to be a new 'good' of 5,600 kWh (house) and 3,600 kWh (GSHP).
  I don't expect to get the GSHP down to as little as 3,325 kWh (the amount of our PV roof last year) because our Sunboxes will have their anniversary in mid March, and therefore the warmer summer figures will be working under similar conditions.

Sunboxes and Mirrors
  As for the Sunboxes... based on their averages of last year, we should have reached 3,000 kWh earning by now, but this spring has been a combination of cloudy and warmish weather, just what is needed to stop them working!
   We should make 2,950 kWh by March 7, the anniversary - close, but not on the nail! Blaise made me turn the Sunboxes off for three days, May 13-15 2010 and these were unfortunately some of the sunniest days of the month with very high scores on the PV roof. We did this to record performance to the datalogger without the influence of the Sunboxes. These would have made us the extra 40-50 kWh we needed to make the magic figure of 3,000 kWh in a year.
    I am often being encouraged to do things like 'take the mirrors off one box and fit an extra energy flow meter to each box, so that they can be compared.' I don't really want to miss out on any of the free summer energy, just to prove a point to those who do not believe that reflectors help - my hunch is that they do.
    Also, and without doubt in my mind... if we had had the mirrors from the start it would have been a different story - a much higher score. Prior to the mirrors' installation, the Sunboxes averaged 1.1 kW from March till September 2010. Since then, with the mirrors installed, they have averaged 1.91 kW in Oct'10-New year, and 1.92 kWh from New year to the end of Feb'11. Although I know that this is a Winter semester, it's clear from their behaviour that they have caused an improvement, and I am sure that the Summer figures will be good enough to enable us to hit the 3,000 kWh annual target next year.


  1. Sorry David - I once again have to disagree with your conclusion on the mirrors. The heat yield increase will due to the fact that the heat pump is running sending cold glycol through the sunboxes, hence an increase temperature difference and thus greater ability to 1)Take heat from the air and 2) Increase the irradiance gain by having a pipe wall tempertaure less than the air tempertaure.

    Think about it another way, can the mirrors really make an 80% increase? especially an increase over the summer months. Remember the sun may be low in winter but it also travels through a greater volume of atmosphere and the irradiance level is consequently much less- so even if it was possible for the mirrors to provide a doubling, then it couldnt have done in this time window as the irradiance is much reduced in any case before it even hits the panels or mirrors.
    Additionally can you claim that the irradiance from the mirrors is nearly equal to that incident on the panels already? only optimised focused mirrors could get anywhere near this performance.

    sorry David - the effect needs quantifying to claim this. and I feel the effect is more to do with the heat pump running in the winter. Do you have a pyranometer to do a quick test a some point? We could place it in the centre of the sunbox and then just put some covers over the mirrors... i.e. just a test taking the time it takes to cover the mirrors.????

  2. HI Chris, Thanks for your continuing interest in this, and thanks for chatting on our stand at EcoBuild. I take on board what you say, and it will not be long before we know. The Sunbox anniversary is this month, and we can begin to do direct comparison in similar weather conditions.

    At the time I installed, it was difficult to test the reflectors in matching weather conditions as they were not all added at once... the top mirrors in September, the foot mirrors in October, the central mirrors in Nov, and the corners in December. With each addition, there seemed an improvement - the highest solar capture of the entire year was in October shortly after the foot mirrors were added.

    Re your pyranometer question. I do have a solar meter on the PV roof, and have results stored for every day going back to 2009, but they are on the east facing roof, not the same azimuth as the sunboxes.

  3. One possible reason Chris, is that I changed the Delta-T to 6 degrees (having tried 3, 4, 5, and 6) . That means it goes less often, but when it does there is a more significant download (i.e. more kW). It appears to capture more, but I also moved the pump to middle speed, so it chills the air in the Sunbox quicker and perhaps causes it to turn off more quickly. Overall, that has meant fewer operating hours, and overall, a poorer harvest, so I have reduced delta-T back to 5 degrees.

  4. Dear Chris, 7 and 8 Mar of 2010 were also extremely sunny, with PV captures of over 14 kWh, higher than in 2011. But the Sunbox capture in 2011 (with mirrors) was significantly higher, 39kWh in two days instead of 29 kWh. Let's hope this continues!

  5. Well, Chris, the lower and side mirrors are gone. This has been a good summer, as evidenced by our record best ever annual PV capture. This has nothing to do with mirrors or lifestyle economies. So the Sunbox figures are high too, but we need to have more cold weather before I know whether the more insulated box construction works better than having a clear enclosure or none. Im still waiting to hear some figures on your solar capture with your solar-focus panels, surely you have simplified weekly or monthly summaries?


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